British light-heavyweight Buatsi earned a win yet unimpressive beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a tight 10-round bout at the boxing arena.
But the challenger had his moments, delivering effective clean blows in a fight where the techniques never quite clicked, with excessive holding marring the contest.
The scoring judges had it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest display but what do champions do? They win," Buatsi stated in the arena as he was greeted with negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts thought Parker, thirty-one, merited the victory but the British boxer experienced the second setback of his 28-fight career.
The Champion Earns Victory in Uneventful Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He opened tentatively, with neither fighter prepared to overcommit in a calculated early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who utilized a tight guard to barely take the more effective periods.
The crowd's patience wore thin amid constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
Parker closed well, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Motivates Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a excellent local rivalry contest to secure the European light-heavyweight championship.
The venue grew hushed for a profoundly moving tribute prior to the bout as a bell ceremony sounded in remembrance of Manchester's own Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the final chime faded, the quiet transitioned to a chant of "only one Ricky Hatton".
The action inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the second round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was marked up quickly, significant bruising forming under his face as the fighters exchanged hard punches.
The challenger, 27, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
The victor edged the decision with scoring tallies of draw, 115-113 and 115-112, but the two athletes were persistent and brave, exactly what their hero Hatton represented.
